The present invention is directed toward a ferrite system concerning switching, attenuating, varying the phase of radio frequency waves, and varying the amplitude of radio frequency waves. More specifically, the present invention concerns a ferrite system for phase modulation, phase shift keying, or phase shifting that can be used in microwave communications equipment, such as transmitters, receivers, and phased array antennas.
The prior art has typically used capacitive coupling inherent in the base-collector junction of a transistor to provide phase modulation of a radio frequency carrier. FIG. 1 illustrates phase modulation using the inherent capacitive coupling of a base-collector junction. A radio frequency (RF) oscillator 50 applies a carrier with phase A to the base of a transistor 60. The transistor 60, in Class C amplification, amplifies when both the audio/data frequency input 52 and the RF oscillator input 50 are sufficiently positive to forward bias the base-emitter junction. The transistor 60 also amplifies when either the audio/data frequency input 52 or the RF oscillator input 50 is sufficiently positive to forward bias the base-emitter junction. The phase C at the collector of transistor 60 is 180 degrees lagging the phase A at the base of the transistor 60 because the transistor 60 is in a common-emitter amplifier configuration. The capacitive base-collector junction has a base-collector capacitance 64 that is reduced in response to increased reverse bias on the base-collector junction of transistor 60. If the resonant circuit 66 has a relatively high quality factor (Q), approaching that of a typical crystal, then the slight capacitive variations of the base-collector junction will detune the resonant circuit 66 from resonance, resulting in phase modulation. Detuning can occur through the collector-emitter path to ground via the base-collector capacitance 64. The degree of phase shift B is controlled by the magnitude of the audio/data frequency input 52 applied to the base of the transistor 60 because the audio/data frequency input 52 varies the value of the base-collector capacitance 64; hence, varies the capacitance of the resonant circuit 66 as well. The aggregate phase at the output terminal 62 comprises components of phase B and phase C.
While the capacitive coupling or reactance modulation technique works well below microwave frequencies (i.e. below approximately 0.9 GHz), at microwave frequencies semiconductor manufacturers strive to reduce the base-collector capacitance of transistors to increase the maximum operating frequencies of radio frequency (RF) transistors. Microwave radio frequency (RF) transistors employ special junction geometries and exotic semiconductor materials to approach the ideal base-collector capacitance of zero picofarads. To phase-modulate a microwave RF carrier frequency, prior art devices typically phase-modulate a low-frequency carrier (i.e. VHF or UHF) and then multiply the carrier up to the desired microwave frequency. The disadvantage of frequency multiplication is that unwanted harmonics are produced, and elaborate filtering is needed to remove these harmonics to comply with Federal Communications Commission (FCC) emission standards. Thus, a need exists for a phase modulator which operates independently of the base-collector capacitance of the transistor and which can directly modulate a microwave frequency carrier.